U.S. patent number 9,039,595 [Application Number 14/162,599] was granted by the patent office on 2015-05-26 for control systems for rotary blood pumps.
This patent grant is currently assigned to Thoratec Corporation. The grantee listed for this patent is Thoratec Corporation. Invention is credited to Peter Joseph Ayre, Lee Thomas Glanzmann, Nicholas Oliver Von Huben.
United States Patent |
9,039,595 |
Ayre , et al. |
May 26, 2015 |
Control systems for rotary blood pumps
Abstract
The present invention generally relates to a control system for
a rotary blood pump adapted to move blood in a patient. The control
system comprises a means for measuring and varying the speed of the
pump and a means for measuring the pulsatility index of a patient,
and the control system is adapted to maintain the pulsatility index
at or near a predetermined value by varying the speed of the pump.
The pulsatility index is derived from the amplitude of the actual
pump speed over a predetermined time period. Optionally, also, the
control system can calculate the second derivative of instantaneous
speed of the rotary blood pump and use the calculation of the
second derivative of instantaneous speed to detect a suction event,
and help prevent it.
Inventors: |
Ayre; Peter Joseph (Crows Nest,
AU), Glanzmann; Lee Thomas (Darlington,
AU), Von Huben; Nicholas Oliver (Bexley North,
AU) |
Applicant: |
Name |
City |
State |
Country |
Type |
Thoratec Corporation |
Pleasanton |
CA |
US |
|
|
Assignee: |
Thoratec Corporation
(Pleasanton, CA)
|
Family
ID: |
38174742 |
Appl.
No.: |
14/162,599 |
Filed: |
January 23, 2014 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20140142367 A1 |
May 22, 2014 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
12468951 |
May 20, 2009 |
8657733 |
|
|
|
11592354 |
Nov 3, 2006 |
|
|
|
|
Foreign Application Priority Data
|
|
|
|
|
Nov 4, 2005 [AU] |
|
|
2005906123 |
|
Current U.S.
Class: |
600/17 |
Current CPC
Class: |
A61B
5/4836 (20130101); A61M 60/50 (20210101); A61B
5/686 (20130101); A61B 5/0215 (20130101); A61B
5/14535 (20130101); A61M 60/148 (20210101); A61M
60/562 (20210101); A61M 60/205 (20210101); A61M
2205/3334 (20130101) |
Current International
Class: |
A61N
1/362 (20060101) |
Field of
Search: |
;600/16-18 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1 354 606 |
|
Oct 2003 |
|
EP |
|
WO 01/05023 |
|
Jan 2001 |
|
WO |
|
WO 01/72352 |
|
Oct 2001 |
|
WO |
|
WO 03/057280 |
|
Jul 2003 |
|
WO |
|
WO 2004/028593 |
|
Apr 2004 |
|
WO |
|
WO 2005/051838 |
|
Jun 2005 |
|
WO |
|
Other References
Ayre et al., "Identifying physiologically significant pumping
states in implantable rotary blood pumps using non/invasive system
observers", Proc. of 25.sup.th Annual Inter. Conf. of IEEE, pp.
439/442 (2003). cited by applicant .
Office Action for U.S. Appl. No. 12/942,908, mailing date May 6,
2011, 12 pgs. cited by applicant .
Response to Office Action for U.S. Appl. No. 12/942,908, mailing
date Aug. 8, 2011, 6 pgs. cited by applicant.
|
Primary Examiner: Getzow; Scott
Attorney, Agent or Firm: Squire Patton Boggs (US) LLP
Claims
The invention claimed is:
1. A method, implemented on a controller, for controlling a blood
pump, comprising: calculating a pulsatility index (PI) based on a
speed of the pump; deriving pulmonary capillary wedge pressure
(PCWP) based on the calculated PI; estimating a flow rate through
the pump based on the derived PCWP; calculating a preferred flow
rate based on a theoretical Starling-like response and the derived
PCWP; comparing the estimated flow rate to the preferred flow rate;
and adjusting the pump speed based on the comparison.
2. The method of claim 1, wherein the deriving of the PCWP
comprises using expert data acquired during treatment of a heart
failure patient.
3. The method of claim 1, wherein the deriving of the PCWP
comprises using a look-up-table (LUT).
4. The method of claim 1, further comprising modifying the flow
rate estimating using expert data.
5. The method of claim 4, wherein the expert data is a member
selected from the group consisting of measured haemocrit (Hct),
measured PCWP, and a combination of the same.
6. The method of claim 5, wherein the Hct is derived from pathology
testing.
7. The method of claim 4, wherein the modifying the flow rate
estimating further comprises: estimating another flow rate based on
Hct, power, and pump speed; comparing the another flow rate to the
estimated flow rate based on the derived PCWP; and adjusting the
estimating the flow rate based on a difference between the
estimated flow rate based on the derived PCWP and the another flow
rate.
8. The method of claim 7, wherein the calculating a PI; the
deriving PCWP; the estimating a flow rate; the adjusting the
estimating flow rate; the calculating a preferred flow rate; the
comparing the estimated flow rate and the preferred flow rate; and
the adjusting pump speed are repeated for each of a plurality of
cycles.
9. The method of claim 1, wherein the adjusting pump speed
comprises setting a pump speed such that the PI is within the range
of about 20 to about 45 units.
10. A control system for a blood pump having an impeller,
comprising: a commutation module for sending a drive signal to
rotate the impeller; a sensor configured to receive an input
indicative of or measure a speed of the pump; and a controller
device for generating a control signal to adjust the speed of the
impeller, the controller device configured to: calculate a
pulsatility index (PI) based on a speed of the pump; derive
pulmonary capillary wedge pressure (PCWP) based on the calculated
PI; estimate a flow rate based on the derived PCWP; calculate a
preferred flow rate based on a theoretical Starling-like response
and the derived PCWP; compare the estimated flow rate to the
preferred flow rate; and wherein the control signal is based on the
comparison of the estimated flow rate to the preferred flow
rate.
11. The control system of claim 10, wherein the sensor is included
with the commutation module and the sensor measures the speed of
the pump by back EMF detection.
12. The control system of claim 10, wherein the controller device
includes a first module configured to receive the calculated PI and
derive the PCWP; a second module configured to receive the derived
PCWP and calculate the preferred flow rate; and a third module
configured to receive the preferred flow rate and the estimated
flow rate and perform the comparison between the estimated flow
rate and preferred flow rate.
13. The control system of claim 12, further including a fourth
module configured to estimate another flow rate based on haemocrit
(Hct), pump power and pump speed, and wherein the estimated flow
rate received by the third module is the estimated flow rate based
on the derived PCWP and modified by the another flow rate.
14. The control system of claim 10, wherein the controller device
is further configured to estimate another flow rate based on expert
data selected from the group consisting of measured haemocrit
(Hct), measured PCWP, and a combination of the same.
15. The control system of claim 10, further including expert data
selected from the group consisting of measured haemocrit (Hct),
measured PCWP, and a combination of the same.
16. The control system of claim 15, wherein the expert data
includes a plurality of measured PCWP values for corresponding PI
values.
17. The control system of claim 15, wherein the expert data
includes Hct derived from pathology testing.
Description
TECHNICAL FIELD
The present invention relates to improvements to control systems
for a rotary blood pump.
BACKGROUND ART
To treat cardiac insufficiency or failure, heart assist devices
have been used to assist the heart of a patient. These heart assist
devices include various pumping devices. A high level of success
has been attributed to a particular group of heart assist devices
called rotary blood pumps.
In the past, the rotary blood pumps have used control systems which
set the pumping speed at a constant rate. This constant rate would
not change for the physiological demands of the patient. Therefore,
if a patient was exercising the physiological demands for increased
blood supply would not be offset by a matched increased pulping
rote or speed of the rotary blood pump.
Therefore, there is a need for a control system that allows a
rotary blood pump to match the physiological needs of a
patient.
Rotary blood pumps also usually provide a continuous flow which is
additionally pulsed by the residual function of the patient's
heart. Rotary blood pumps operating at predetermined fixed pumping
rate often tend to over-pump or under-pump blood from the ventricle
depending on the physiological needs of the patient and this may
lead to deleterious effects on the patient including, but not
limited to, suction events or ventricular collapse. Suction events
occur where the pressure within a ventricle is less than the
intrathoracic pressure around the heart. The net result is a
partial or complete collapse of the ventricle.
The present invention aims to or at least address or ameliorate one
or more of the disadvantages associated with the above mentioned
prior art, or to provide a useful alternative.
DISCLOSURE OF THE INVENTION
In accordance with a first aspect, the present invention consists
of a control system for a rotary blood pump adapted to move blood
in a patient, the control system comprising a means for measuring
and varying the speed of the pump and a means for measuring the
pulsatility index of a patient, the control system adapted to
maintain the pulsatility index at or near a predetermined value by
varying the speed of the pump, and the pulsatility index is derived
from the amplitude of the actual pump speed over a predetermined
time period.
Preferably, the predetermined time period is about 40 milliseconds.
Preferably, the predetermined value is between 20 to 45 units.
Preferably, the control system calculates the second derivative of
instantaneous speed of the rotary blood pump and uses the
calculation of the second derivative of instantaneous speed detect
a suction event.
Preferably, the control system determines imminence of a suction
event based on the stroke work. Preferably, the target speed is
pulsed in cooperation with the heart. Preferably, the control
system includes a selective mode that minimises target pump speed
to achieve forward blood flow through both the pump and aortic
valve, whilst avoiding retrograde flow. Preferably, the selective
mode sets target speed at about 1250 rpm.
Preferably, the control system calculates or detects fell
ventricular pressure with respect to time. Preferably, the control
system uses the pulsatility index to derive preload. Preferably,
the control system maintains the preload within a predetermined
range by adjusting target speed. Preferably, the control system
mimics starling curve responses of a natural heart. Preferably, the
control system uses preload to mimic starling curve responses of a
natural heart.
In accordance with a second aspect, the present invention consists
of a control system for use with rotary blood pumps, wherein the
control system includes a selective mode that minimises target pump
speed to achieve forward blood flow through both the pump and
aortic valve, whilst avoiding retrograde flow.
BRIEF DESCRIPTION OF DRAWINGS
Embodiments of the present invention will now be described with
reference to the accompanying drawings wherein:
FIG. 1 depicts a schematic view of a first preferred embodiment of
the present invention;
FIG. 2 depicts a graph of an example of a relatively normal
starting response of the natural healthy heart;
FIG. 3 depicts a graph of an example data from a theoretical
healthy patient with varying the cardiac outputs compared to
Pulmonary Capillary Wedge Pressure (herein referred to as `PCWP`)
beyond the normal "Starling-Like" response of a typical
patient;
FIG. 4 depicts a graph wherein pump flow has been plotted against a
Pulsatility Index (herein referred to as `PI`):
FIG. 5 depicts a graph demonstrating an example relation between
PCWP to PI;
FIG. 6 depicts a graph wherein Left Ventricular Pressure (herein
referred to as `LVP`) is compared to pump sped over time and
further wherein PI is set at a relatively normal level;
FIG. 7 depicts a similar graph to FIG. 6, wherein the PI is set at
a relatively low level; and
FIG. 8 depicts a similar graph to FIG. 6, wherein a suction event
has occurred.
BEST MODES FOR CARRYING OUT THE INVENTION
In a first preferred embodiment of the present invention, as
depicted in FIG. 1, a control system 1 is used to control the
target speed of a rotary blood pump 2. The rotary blood pump 2 may
be implantable or extracorporeal; and may also be a left ventricle
assist device. The preferred rotary blood pumps 2 for use with the
first embodiment of the present invention are described in: U.S.
Pat. No. 6,227,797 (Watterson et al) or U.S. Pat. No. 6,866,625
(Ayre et al) and the descriptions of these inventions are included
herein.
The control system 1 may include several steps or modules to
control the target speed of the rotary blood pump 2. Preferably,
the control system 1 includes a commutation module 3. The
commutation module 3 provides the rotary blood pump 2 with an
electromagnetic drive signal to rotate a rotor or impeller (not
shown) positioned within the rotary blood pump 2. The commutation
module 3 also may detect the actual pumping speed or the actual
speed of rotation of the impeller within the rotary blood pump 2
using back EMF detection.
The actual pumping speed may then be used by the control system 1
to derive or calculate a Pulsatility Index (`PI`) and this is
depicted as step 4 in FIG. 1.
The control system 1 then may also derive or calculate Pulmonary
Capillary Wedge Pressure ("PCWP"), which also can be referred to as
"preload", from a look-up table of set values or from an equation.
This is shown in FIG. 1 as step 11. At this step 11, the control
system 1 may a receive additional external input from data acquired
in an Intensive Care Unit ("ICU") environment and is depicted in
step 10.
Preferably, the control system 1 then calculates the most preferred
pump flow rate derived or calculated from an ideal or theoretically
"Starling-Like" response (see below) and this is depicted in FIG. 1
as step 7.
At step 7, the control system 1 may also receive additional expert
data 8. This expert data 8 is generally data entered by an expert
or medical professional and may generally include data such as
haemocrit (herein referred to as "Hct") levels derived from
separate blood pathology testing. If the control system 1 receives
this expert data 8 at step 7, the control system 1 proceeds to step
9. Step 9 allows the control system 1 to calculate flow derived by
Hct, power and pump speed.
If no expert data 8 is received by the control system at step 1,
the control system 1 then proceeds to step 6. During step 6, the
control system 1 compares the actual pump flow with the desired
pump flow and adjusts the pump speed signal, accordingly.
Step 5 allows the control system 1 to determine whether a suction
event has or is about to occur and further allows the control
system 1 to reduce the pumping speed signal to avert the suction
event, where necessary.
Step 3 in the control system 1 allows the control system 1 to
convert the pump speed signal into a commutation signal to drive
the rotation of the impeller within the rotary blood pump 2.
Generally, a normal healthy heart follows the relationship depicted
in the graph 24 of FIG. 2. Wherein, the output of the heart or
Cardiac Output 25 (herein referred to as "CO") behaves according to
the PCWP 22 for a given Mean Arterial Pressure 23 (herein referred
to as "MAP" or "afterload"). This relationship is known as the
"Frank-Starling Law of the heart" or "Starlings law of the heart"
to persons working in this area. A relatively normal healthy heart
is known to have a "Starling-like" response and may have a
relatively normal range of MAP 23 of between 70 mm Hg to 120 mm Hg.
There exists a close relationship of curves representing CO 25 as a
function of PCWP 22. Data from patients experiencing heart failure
may generate a family of curves wherein the CO 25 is reduced for a
given PCWP 22.
Preferably, as depicted in FIG. 1, the control system 1 may vary
the pump output or pump speed using a control algorithm. The
control system 1 cooperating with the rotary blood pump 2 may
actively restore the CO 25 of the heart failure patient to
relatively normal levels for given PCWP 22 values.
This may be non-invasively achieved by observing the pulsatility
amplitude or PI 30 of pump speed and thereby allowing a relatively
unique relationship between PI 30 and PCWP 22 to be able to be
established for each patient. FIG. 4 depicts several examples of PI
30 being mapped against p/flow 29 (which is effectively the
equivalent of blood flow through the blood pump 2). Preferably, the
pump 2 is being driven at a pumping speed within the LV preferred
pumping zone 28, which defines the optimal pumping conditions for
the pump 2 used in this embodiment. Lower zone 26 depicts a region
wherein the pumping speed is relatively too low and upper zone 27
depicts a region wherein the pumping speed is relatively too
high.
Relationships for PI 30 and PCWP 22 can be established for each
patient by incrementing pump speed and recording PCWP 22 using
standard measurement techniques while the patient is in intensive
care. A diagram demonstrating this unique relationship is shown in
FIG. 5.
PI 30 may be derived from instantaneous speed of a rotor within the
rotary blood pump 2. PI 30 may therefore be used in feedback loop
in the control system 1 to produce relatively normal response
curves for each patient. The pressurisation of the Left Ventricle
(herein referred to as "LV") and its influence of rotor speed is
shown in FIG. 6 (normal pulsatility) and FIG. 7 (low
pulsatility).
PI 30 may be a general indication of the pulsatility of blood
flowing through the rotary blood pump 2. Preferably, clinicians or
patients may set a pumping speed of the rotary blood pomp 2 by
inputting a target speed into the control system 1. This original
target speed may be interpreted by the control system 1 as either a
single set point or a desired operating range for pump speed
(example preferred ranges 28 in FIGS. 3 & 4). The amount of
blood delivered from the LV to the pump varies across the cardiac
cycle depending on several factors (including but not limited to
preload, after-load, cardiac contractility, stroke volume, and
heart rate). As a result, the rotor speed may vary significantly
with the cardiac cycle. Preferably, PI 30 may be calculated by
measuring the amplitude of these variations over one or several
cardiac cycles and scaling this by a constant to produce an index
between 0 and 100. Preferably, the normal range for PI 30 is set to
about 20 to 45 units.
Increases in LV flow rates generally lead to increases in blood
reaching or delivered to the rotary blood pump 2. As Left Ventricle
Pressure 31 (`LVP`) increases, aortic pressures may become
relatively negative and the pressure differential across the rotary
blood pump 2 may decrease. This decrease in pressure across the
pump 2 may also lead to a reduction in flow within the pump 2.
Increased fluid loads may cause the rotor in the pump 2 to slow
slightly. When LVP 31 is reduced, the rotor within the pump 2 may
slightly increase in rotation speed. Amendments to the actual pump
speed 32 may alter the PI 30, as the effectiveness of ventricular
unloading is altered. Additionally, when the aortic valve no longer
opens (because of relative pressure or mechanical failure), PI 30
may be reduced.
Increasing the target speed may cause maximum LVP 31 to drop as the
LV is unloaded more quickly and aortic flow decreased. The pomp
speed waveform dampens (less variation in speed across the cardiac
cycle) and poise pressure lowers and disappears.
With static system performance parameters increasing target speed
may lead to a decrease PI 30. This situation is demonstrated in the
graph shown in FIG. 7.
Decreasing the target speed may allow the LV more time to fill and
thus permits it to contract effectively and leads to improved
pulsatility. With a relatively static system, performance
parameters set for a decreasing target speed may lead to increase
PI 30.
A relatively low PI 30 may suggest that the LV is not adequately
contracting and this may be a result of the pump target speed being
too high. This generally results in over-pumping blood 26 from the
LV.
High PI 30 may generally indicate that there is increased
pulsatility of flow through the pump. In situations where a high PI
30 is being experienced, pump target speed may need to be increased
to more efficiently offload the ventricle. A high PI 30 may also
occur where the pump target speed is too high relative to the
amount of blood being delivered to the pump. The ventricle walls
may collapse may lead to temporary increase in PI 30. These types
of situations may be indicative of underpumping 27 of the LV.
Preferably, PI may be calculated by the following formula:
PI=(Maximum pump speed-Minimum pump speed)/Pulsatility scaling
factor
In the first embodiment of the present invention, PI 30 is
calculated by the averages of the last 5 speed samples executed at
40 millisecond intervals. The preferred array stares the last 100
averaged speed samples for comparison. Instantaneous speed values
32 may be updated at a rate dependant on the pomp speed (i.e. the
speed samples may be collected every 8.sup.th transition of the
pump speed signal from low to high) but where the sampling
preferably occurs every 40 milliseconds. PI 30 may also be detected
in respect of cardiac cycles and the preferred control system may
detect PI 30 over 4 cardiac cycles (assuming average rate is 60
bpm=5 secs)
For each patient a relationship established between PI 30 and PCWP
22 initially through invasive measurement of PCWP 22. This
relationship is produced between the desired CO 25 of the patient
as a function of PI 30.
The most preferred rotary by pump 2 includes a relatively flat flow
rate Vs pressure curve or relationship for any given pump speed, a
characteristic of centrifugal pumps but particularly with those
which utilise a hydrodynamic bearing in their design. This
characteristic further assists in providing a "Starling-Like"
response, when used in combination with the present embodiment.
In this first preferred embodiment, the actual pump speed may be
used to derive PI 30 by the control system 1. The actual pump speed
signal provides a relatively low noise signal for use by the
control system 1 (especially when compared to the signal of the
current drawn by the motor of the pump) and actual pump speed also
does not include other variables within it's signal composition
(the signal generated in relation to current or power used by the
motor of the pump may typically include other variables such as
preload values, pressures and left atrial pressures).
A person working in this area may also appreciate that the
automatic calculation of LVP 31 and PI 30 by the control system 1
may be replaced with implantable sensors (not shown). These
implantable sensors may detect the input data directly and feed
back this data directly to the control system 1 and may be
implanted in the patient. The control system 1 may then amend the
target pump spend accordingly based on these detected inputs.
The control system may also provide a pulsed target speed to the
rotary blood pump 2. This pulsed target speed may attempt to
emulate or enhance the pulsing of the normal patient's heart The
pulsing target speed allows the rotary blood pump 2 to be
continuously operated to avoid thrombogenesis. The pulsing target
speed generally occurs within a range of between 1250-3000 rpm. The
pulsing of target speed may also be timed with the detected heart
rate of the patient. Additional heart rate sensors (not shown) may
directly detect the patient's heart rate in real-time and feed this
information back to the control system for timing adjustment to the
pulsing target speed.
The control system 1 may also include a selective mode called `CPR
mode` (not shown), which can be selectively activated by
clinicians. CPR mode may be activated by a software interface
working with the control system 1 and preferably is activated when
the patient requires external CPR. The control system 1, when in
CPR mode, reduces the pump target speed range to between 1250-1800
rpm. The preferred target speed of the control system 1 in CPR mode
is about 1250 rpm. In CPR mode, the forward flow of blood through
both the pump and aortic valve is maintained and retrograde flow
back into the ventricle is avoided, provided that the external CPR
generates a Mean Arterial Pressure (`MAP`) of at least 40 mm Hg. In
CPR Mode, the rotary blood pump continues to run at a minimum speed
so that the risk of thrombogenesis proximal to the rotary blood
pump or patient's heart is reduced or eliminated. Preferably, this
minimum speed is low enough to allow the ventricle time in fill
from the left atrium, but not from the aorta via the pump conduit.
This should lead to resolution of the suction event and more
effective contractions during periods of haemodynamic
compromise.
Additionally, if the control system 1 detects the imminence of a
suction event, the control system 1 may preferably automatically
activate CPR mode reducing the pump speed to a default value or a
set value. The effect of the activation of the CPR mode is that the
target speed of the pump is quickly reduced to a minimum safe
operating speed. The minimum safe operating speed achieves all of
the aforementioned advantages of CPR mode.
Additionally, the control system 1 may also calculate the second
derivative of instantaneous sped (not shown) of the rotary blood
pump 2 and then use this calculated second derivative of speed of
speed to predict the imminence of a suction event. The second
derivative of instantaneous speed may show sharp angular peaks 33
as shown in FIG. 8, when a suction event may be imminent. These
peaks are generally the result of the LV wall being rapidly pulled
towards the septal wall of the heart at the end of LV ejection. The
sudden reduction in pump flow unloading the pump's rotor casing the
speed to rise rapidly. This situation accentuated by low blood
volume in the LV caused by low PCWP 22. Detection of these sharp
angular peaks 33 also allows the detection of a suction event. The
control system 1 may then reduce the target speed of the rotary
blood pump 2 to remedy or at least partially avert the imminent
suction event.
The above descriptions detail only some of the embodiments of the
present invention. Modifications may be obvious to those skilled in
the art and may be made without departing from the scope and spirit
of the present invention.
* * * * *